(1) Field of the Invention
The present invention relates to novel optically-active aliphatic .alpha.-halogen substituted carboxylic acid 4'-(4-alkoxybenzyloxy)biphenyl thioester compounds defined by the general formula: ##STR3## wherein R represents a n-alkyl radical having C.sub.8 to C.sub.15, preferably C.sub.8 to C.sub.10 carbon atoms, C* represents an asymmetric carbon atom, X represents a halogen atom and R' represents ##STR4## and process for the preparation thereof.
In particular, the present invention relates to novel compounds usable as a base material or a dopant for liquid crystal blending to improve the properties of LCD, and a process for the preparation thereof.
(2) Description of the Prior Art
Generally, a liquid crystal phase having an optically-active group has been well-known but the fact that a portion of a smectic phase shows ferroelectricity was first discovered by R. M. Meyer.
Since this ferroelectric liquid crystal (FLC) has desired properties, such as fast response time and bistability, many specialists have taken an interest in FLC in order to apply it to color TVs, computer terminals, electrically-optical modulators, etc., utilizing such properties.
However, studies on FLC have been stagnated due to the difficulties of its applicability. After a surface stabilized ferroelectric liquid crystal (SSFLC) was made public by Clark Lagerwall et al., in 1980, it has become a more important material. Accordingly, various theoretical, experimental and chemical studies on FLC have been carried out in its field of application.
Since an FLC material having fast response time and bistability and being a smectic phase (Sc*) in a broad temperature range is necessary in order to apply an FLC to liquid crystal display elements, several classes of an FLC substance have been synthesized to meet such conditions. In spite of all such efforts, with conventional FLC substance, there is much left to be desired.
An FLC substance of cinnamate and phenyl benzoate series having the chain of ##STR5## was disclosed by P. Keller, et al.. Recently, K. Yoshino, J. Dijon, C. Bahr and G. Anderson, et al. have studied biphenyl and benzoate series having a chiral center which can be obtained from optically-active .alpha.-amino acid. From the results, they explained that these compounds show high spontaneous polarization by introducing two adjacent asymmetric carbon atoms into the side of dipole moment and the magnitude of the spontaneous polarization depends on the size of an atom bonding to a carbon atom and the polarity.
In 1988, B. Shivkamur proved, through his studies on p-n-alkoxy-.alpha.-methyl cinnamic acid ester derivatives, that a halogen substituent of an asymmetric carbon atom induces high spontaneous polarization by introducing a chlorine substituent into .alpha.-asymmetric carbon atom of optically-active group.
It has been found that biphenyl series are chemically stable and an ester bridge between phenyl groups has a tendency to lower the transition temperature. It has also been found that two adjacent asymmetric carbon atoms at the side of dipole moment and a halogen substituent of an asymmetric carbon atom induce high spontaneous polarization.
Thus, in a ferroelectric liquid crystal substance:
1. a rigid core PA0 2. a flexible end chain PA0 3. the transverse dipole PA0 1. it should exhibit a smectic phase in the practical temperature range; PA0 2. it should be chemically stable; PA0 3. its spontaneous polarization should be high; and PA0 4. its bistability should be good and response speed high. PA0 reacting an optically-active .alpha.-amino acid with HF/pyridine or HCl/NaNO.sub.2 in a solvent of distilled water under an ice bath for 7 to 10 hours to provide an optically-active aliphatic .alpha.-halogen substituted carboxylic acid; PA0 reacting said optically-active aliphatic .alpha.-halogen substituted carboxylic acid with a p-(4-hydroxyphenyl)-benzoic acid in a solvent of pyridine at a temperature of from 60.degree. to 100.degree. C. for 8 hours to provide an optically-active aliphatic .alpha.-halogen substituted carboxylic acid p-(4-carboxy)biphenyl; and PA0 reacting said optically-active aliphatic .alpha.-halogen substituted carboxylic acid p-(4-carboxy)biphenyl with an alkoxythiophenol defined by the general formula: ##STR7## wherein R has the same meaning as previously defined, by heating under reflux at a temperature of about 80.degree. C. for about 12 hours in a solvent of pyridine.
1) is a primary basic anisotropic unit, and PA1 2) is a complex cyclic group as well; PA1 1) is an essential member to form liquid crystals, PA1 2) helps to form a declined smectic phase, and PA1 3) has an effect on the transition temperature by the size of its chain; and PA1 1) should be as large as possible because of it's effect on formation of an FLC mesophase, and PA1 2) should be close to a chiral center to induce high spontaneous polarization.
On the basis of the above, a molecular design is constructed as follows: ##STR6##
By these basic conditions, the properties of substantially practical FLC substance are as follows: